I've done some modifications to the mp4clip tool (part of the MPEG4ip software package) to do key frame
accurate cutting/clipping of mp4 files. In general it seems to work, but from
time to time it corrupts the source (!) files. Need to find time for
debugging.

I'll release the patch as soon I consider it to be used safely. Don't want to be responsible for corrupting someones video collection...